1. Field of the Invention
The present invention relates to an electromagnetic relay and more particularly to the electromagnetic relay being suitable for control of a vehicle-mounted three-phase brushless motor.
The present application claims priority of Japanese Patent Application No. 2004-158951 filed on May 28, 2004, which is hereby incorporated by reference.
2. Description of the Related Art
In recent years, an operating method of power steering for an automobile is changing from a hydraulic type to an electric type with the aim of improving automobile fuel consumption. At present a steering method for controlling a DC (Direct Current) motor is used in electric power steering systems in most cases. However, electric power steering systems using a three-phase brushless motor increases as its application of electric power steering to an automobile having a large piston displacement increases. As a result, a switching device that can control three phases simultaneously or that can control only two phases out of three phases becomes necessary. For example, in order to control the three-phase motor, an electromagnetic relay is becoming very useful which can simultaneously control three phases at a midpoint (connection point) of star connection or can control only two phases. Prerequisites to the control of the electric power steering by the electromagnetic relay are a large current-carrying capability (which allows a current of, for example, 100A to flow) and an interrupting capability (which can interrupt a voltage and a current of, for example, 100A at 14V.) to provide torque for the motor. Additionally, in response to a high rate at which components are made electrical and electronic in an automobile, it is required that the electromagnetic relay is miniaturized further.
A conventional electromagnetic relay is disclosed in Japanese Patent Application Laid-open No. 2002-329447 in which a vehicle-mounted electromagnetic relay can be so configured as to be smaller in size. However, conventionally, a single electro-magnetic relay has not yet been known which has a current-carrying capability (which allows a current of, for example, 100A to flow) and an interrupting capability (which can interrupt a voltage and a current of, for example, 100A at 14V) being large enough for one electromagnetic relay to be able to control one three-phase brushless motor for electric power steering.
Also, at present, an electromagnetic relay designed to control the three-phase brushless motor is of a type having only one circuit with one ordinarily-open contact (which is called “1 FORM A-type”). One electromagnetic relay is used in each of, at least, two circuits out of three circuits to be used for control of the three-phase brushless motor or a plurality of electro-magnetic relays is used, in order to divide a current within the one circuit to be used for the control of the three-phase brushless motor, depending on a performance capability of the electromagnetic relay.
In current automobiles, due to miniaturization, high-density mounting, and cost reduction in various electronic components, the rate at which components are made electrical and electronic has increased. In other words, a present state is that the electromagnetic relay to be mounted on an automobile requires further miniaturization of its components, further improvement of accuracy in assembling the components, and further increases in productivity of the components, and cost reduction in the components.
When a plurality of electromagnetic relays each having only one circuit with one ordinarily-open contact is used for the control of the three-phase brushless motor, a ratio of the electromagnetic relays to occupy a space within a device using electrical and electronic components becomes greater, use of a plurality of electromagnetic relays does not meet a customer demand to provide a space-saving structure as much as possible for the devices made up of electrical and electronic components. Therefore, it is necessary that a single electromagnetic relay has two or more circuits each having the ordinarily-open contact, and a ratio of the single electromagnetic relay to occupy a space within a device using electrical and electronic components is smaller than a ratio of a plurality of electromagnetic relays, each having one circuit with one ordinarily-open contact, to occupy a space within the device using electrical and electronic components. In such an electromagnetic relay as described above, it is necessary that the ordinarily-open contact between the two circuits is kept in an insulated state and that an insulating member called a “card” is inserted between the two circuits. Additionally, in order to allow a current of 100A to flow, it is necessary that resistance across terminals is made as low as possible so that the resistance across the terminal is 1 m □ or less. To achieve this, by shortening a length of a current path for a current flowing through the electromagnetic relay as much as possible and by increasing a cross-sectional area of a current path as much as possible, resistance of a conductor must be low. Also, it is necessary that a contacting strength by which contacts are pressed against one another is made as great as possible and contact resistance among contacts is made low and is stable. It is also necessary that the electromagnetic relay can withstand shock and vibration of an automobile. Furthermore, the electromagnetic relay plays a role of breaking circuit connection in an abnormal state, and therefore it is required for the electromagnetic relay to be capable of interrupting a current of about 100A at 14V DC (Direct Current) as interrupting capability.